Heat Shock and Molecular Chaperones

 
Protein folding takes place inside cells at their normal temperature. This temperature is 37°C in the case of mammals. If cells encounter higher temperatures, their proteins become unfolded since the minimal energy conformation at one temperature isn't the same minimum at a another temperature.

The difference in temperature isn't large. Many mammalian proteins become unstable at 42°C. This is a temperature encountered with high fever but it's also common in skin cells that have been exposed to the sun or the water in a hot tub. Temperature differences are even more common in species that do not regulate their temperature (e.g., plants, fungi, bacteria, invertebrates).

All living cells have defense mechanisms that protect against exposure to high temperature. They produce special proteins called heat shock proteins in order to recover damaged proteins that have unfolded at the high temperature. Many of the heat shock proteins are molecular chaperones. These are proteins that guide proper refolding of damaged proteins.

Shortly after the discovery of heat shock proteins we learned that these proteins are always present even in cells that haven't been stressed. In other words, molecular chaperones play a role in normal protein folding as well as helping to recover from damage. This role is illustrated in the energy diagram (top left).

One of the pathways to the energy minimum follows the line labeled "B." This traverses a local energy well and proteins can get trapped in this pit. What happens is that they adopt a less-than-optimal conformation but they can't easily unfold and refold to fall down into the deeper well because that would require an increase in energy. It will happen eventually, but it could take a long time. Chaperones help direct folding along the proper pathway and his speeds up the folding process.

Recall from the earlier discussion of How Proteins Fold that folding is an entropy-driven process where the end result is to bury hydrophobic residues in the central core of the protein. Another thing that can happen during folding is that exposed hydrophobic surfaces of one protein can interact with similar surfaces in another protein leading to aggregation. Chaperones can bind to these hydrophobic surfaces and prevent aggregation. This is another way of speeding up folding.

There are many different chaperones. My personal favorite is HSP70 (Heat Shock Protein of relative molecular mass 70,000). This is a protein that's found in every type of living species. It is the most highly conserved protein in all of biology so it's an excellent protein for looking at deep phylogeny. The role of HSP70 as a molecular chaperone is to bind to proteins as they are being synthesized in order to prevent improper folding. It also prevents aggregation.

Another famous chaperone used to be called HSP60 since it was a heat shock protein of M_r= 60,000. It was also known as GroE since it allowed for the growth of bacteriophage λ gene E mutants. Now it's known as chaperonin. Note that the term "chaperonin" refers to a specific molecular chaperone.

Chaperonin is a barrel-shaped molecule consisting of two rings of seven subunits surrounding a central cavity (one subunit is colored green in the image) . The top of the cavity is sealed by cap of seven smaller subunits. The chaperone works by capturing small unfolded proteins in the cavity where the hydrophobic environment encourages rapid folding to the correct conformation. Aggregation is also prevented by keeping the folding protein away from other proteins.

The inside of the cavity is referred to as an Anfinsen cage after Christian Anfinsen, a Nobel Laureate who worked on protein folding (see next Wednesday's Nobel Laureate). The release of unfolded protein is coupled to the hydrolysis of ATP. This is a common feature of most molecular chaperones.